An ink jet recording method is a printing method where printing is performed by causing small droplets of ink to fly out and adhere to a recording medium such as paper. Generally, any of various aqueous dyes dissolved in water, or in water and a water-soluble organic solvent may be used as an ink. It has generally been pointed out that an image formed by ink containing such a water-soluble dye is poor in waterfastness and lightfastness.
On the other hand, ink obtained by dispersing a pigment in an aqueous medium is excellent in waterfastness and lightfastness. For example, an aqueous pigment ink in which a pigment is dispersed in a surfactant or a polymer dispersant has been proposed. In these inks, however, there has been a case of an exponential increase in ink viscosity occurring when increasing the content of a colorant in the ink to raise the printing density of a recorded material. In addition, there has been a case that the need for an excessive amount of a surfactant or a polymer dispersant to stably disperse a pigment in ink led to a decrease in printing stability due to bubble generation and decreased defoaming property.
To solve these problems, a dispersion liquid for a self-dispersing pigment has been disclosed. It is prepared by introducing a predetermined amount or more of surface-active hydrogen or a salt thereof to the surface of the pigment so that it can disperse alone in a water-based solvent even without the presence of a surfactant or a dispersant such as a polymer dispersant.
In JP-A-10-237349 (Patent Document 1), JP-A-8-3498 (Patent Document 2), and International Publication No. 01/94476 pamphlet (Patent Document 3), so-called self-dispersing pigments that do not require any dispersant as described above are reported to achieve the following, when used as colorants in ink: an increase in OD value (optical density) of an image; ease of use because of facilitating the adjustment of viscosity in ink to within a proper range; consideration of the compatibility of the dispersant with various kinds of solvents to be added is not needed; and so on. A hydrophilic functional group, such as a carbonyl group, a carboxyl group, a hydroxyl group, a sulfone group, or an ammonium group, is bonded to the pigment surface directly or indirectly through an alkyl group, an aryl group, or the like.
Furthermore, JP-A-10-95941 (Patent Document 4), JP-A-2001-207089 (Patent Document 5), and JP-A-2000-219832 (Patent Document 6) propose ink jet inks that contain the above self-dispersing carbon black, glycol ethers, and so on.
On the other hand, there is a problem in that an image formed with an ink using such a self-dispersing pigment as a colorant is generally poor in fixability. A recorded material with a sufficient rubfastness may not be obtained because of smudging the image when the image is scrapped by a finger, a line marking pen, paper, or the like. Therefore, to improve the fixability of the self-dispersing pigment to a recording medium, a proposal has been made to add a resin having binding efficacy to a recording medium.
JP-A-2001-81366 (Patent Document 7) and JP-A-2002-80761 (Patent Document 8) propose, as an ink composition containing resin particles, a recording liquid consisting of a colorant, resin fine particles, and water; a recording liquid for an ink printer consisting of a pigment dispersed in water and resin fine particles; and so on.
On the other hand, JP-A-2002-121434 (Patent Document 9) discloses an aqueous color ink set for ink jet recording that contains a black ink and also contains at least a cyan ink, a magenta ink, and a yellow ink as color inks, where the black ink and the respective color inks contain pigments which are self-dispersible in water. In addition, it is also disclosed that a self-dispersing pigment can be prepared by treating, for example, a cyan pigment with sulfanilic acid and sodium sulfite.
Furthermore, JP-A-10-212426 discloses an oxidation-treated carbon black obtained by oxidation treatment of carbon black with an oxidizing agent and a method for manufacturing a carbon black dispersion liquid therefrom (see Patent Document 10). However, there is no disclosure about the surface treatment of pigments for color ink compositions.